1. Field of the Invention
The present invention relates to a semiconductor memory device that stores data in accordance with the presence/absence or the magnitude of a current. More particularly it relates to an improvement in a sense amp circuitry operative to compare a potential on a data line with that on a reference data line for data sensing.
2. Description of the Related Art
An EEPROM is known as an electrically rewritable semiconductor memory device for nonvolatile storage of data. A type of EEPROM can erase multiple memory cells collectively at a time, which is called the flash memory. Such the semiconductor memory stores data in a memory cell in accordance with the presence/absence or the magnitude of a current flowing in it and accordingly employs a sense amp circuit of a current read-out type. A type of such the sense amp often employed compares a voltage caused on a data line based on data read out of a memory cell with a reference voltage on a reference voltage line for data determination.
FIG. 19 shows an arrangement of such the sense amp circuit in the art. A sense amp body 311 comprises a differential amp (opamp) OP. The differential amp OP has two input terminals connected to a sense line SN and a reference sense line RSN, respectively. The sense line SN is connected through a NMOS transistor QN1, or an isolator (clamper), to a data line DL that is led to a bit line BL from a selected memory cell MC. The isolator is located to suppress the drain voltage on the memory cell MC below a certain level. A PMOS transistor QP1, or a current source load, is also connected to the sense line SN. The PMOS transistor QP1 and the isolator NMOS transistor QN1 together configure a cascade amp 310.
To the reference sense line RSN, a reference voltage generator 320 is connected to generate a middle reference voltage between voltages appeared on the sense line SN based on data. The reference voltage generator 320 comprises a reference cell RMC having the same structure as that of the memory cell MC. It also comprises a current source NMOS transistor QN11, which reflects the current flowing through the reference cell RMC to flow a current that is half the current, Icell, flowing through an ON-cell (a cell in a state of data “1”). The current source NMOS transistor QN11 has a drain connected to the reference sense line RSN via the isolator transistor QN2, similar to the sense line SN. A current source load PMOS transistor QP2 is connected to the reference sense line RSN. The data line DL has a relatively large capacitance in general. Therefore, a dummy capacitor CR is connected to a reference data line RDL to match its capacitance with that of the data line DL.
The current path through the reference cell RMC configures a reference cell unit 320a, which includes the isolator NMOS transistor QN4 and the current source load PMOS transistor QP4 serially connected with the reference cell RMC. In order to transfer the reference current, I0, flowing through the reference cell RMC to the current source transistor QN11, the current path through the reference transistor QN10 configures a reference transistor unit 320b. It includes the isolator NMOS transistor QN3 and the current source load PMOS transistor QP3 serially connected with the current source transistor QN11. The current source PMOS transistor QP3 has a drain node N1 connected to a gate of the reference NMOS transistor QN10.
The PMOS transistors QP3 and QP4 configure a current mirror circuit. Accordingly, the current, I1, flowing through the reference transistor QN10 reflects the reference current, I0, flowing through the reference cell RMC. In accordance with a size ratio between the PMOS transistors QP3 and QP4, the current I1 has a constant ratio to the reference current I0. The reference NMOS transistor QN10 and the current source NMOS transistor QN11 also configure a current mirror circuit. Accordingly, a size ratio between them can be determined so that the current, 12, flowing through the current source transistor QN11 has a constant ratio to I1.
This reference voltage generator allows the current source transistor QN11 to flow a current, Icell/2, where Icell denotes a cell current flowing into a selected memory cell in an ON-cell state. As a result, when the sense amp 311 senses a difference between the voltage on the sense line SN and that on the reference sense line RSN, it is possible to determine whether the data is “1” or “0”.
Such the conventional sense amp circuitry causes a problem associated with a lower supply voltage, which is described below. FIG. 20 shows voltage distributions in the sense amp circuit when a supply voltage is set to Vcc=2.7V and when Vcc is further lowered. To achieve a cell current necessary for a read operation, a bit line voltage is required to keep a minimum value of 0.5V. A difference in bit line amplitude between an OFF-cell and an ON-cell has an upper limit determined to 0.3V as required for suppressing the so-called soft-write (a little written phenomenon caused by the read operation). A current source load PMOS transistor is assumed to have a threshold value of Vtp=−0.8V. In this case, if Vcc=2.7V, a sense line amplitude is allowed to have a possible range of 1.1V.
Thus, it is possible to hold an operating voltage sufficient to configure the cascade amplifier as shown in FIG. 19 that includes the isolator interposed between the sense line SN and the data line DL. When lowered to Vcc=1.8V and if the bit line voltage minimum, the bit line amplitude possible range, and the voltage drop corresponding to the threshold value of the current source load PMOS transistor require to keep their voltages unchanged, the sense line amplitude possible range is reduced as shown in FIG. 20. In this case, it is impossible to hold an amplifying operation from the bit line to the sense line. When lowered to Vcc=1.5V, no operating margin can be held.
In consideration of such the situation, the Inventors et al. have previously proposed a bit-line direct-sensing scheme, as a preferable sense amp circuitry capable of responding to a supply voltage lowered below 2V, in which a sense node is connected directly to a data line without locating an isolator. (See: (1) Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE, ICD 200-13; and (2) Atsumi S. et al., “A Channel-Erasing 1.8V-Only 32 Mb NOR Flash EEPROM with a Bit-Line Direct-Sensing Scheme”, ISSCC 2000 Digest of Technical Papers, pp. 276–277 (2000. 2)).
Even if the above bit-line direct-sensing scheme is applied, however, the conventional sense amp circuitry has another problem in the reference voltage generator, which prevents the supply voltage to be lowered. This problem is described using the reference voltage generator 320 in FIG. 19. In order to allow the gate node N3 of the current source transistor QN11 to be kept at a constant potential regardless of the power supply Vcc, the reference NMOS transistor QN10 is required to operate as a pentode. In consideration of this point, a minimum supply voltage, Vccmin, is examined, which allows the reference voltage generator 320 to operate. The isolator NMOS transistors QN3 and QN4 are not considered because they can be removed.
When a drain voltage of Vdn=0.8V is required for the reference cell RMC to flow a current through it, and the current source load transistor QP4 has a threshold voltage of Vtp=−1V, a minimum supply voltage, Vccmin1, at the reference cell unit 320a comes to Vccmin1=Vdn+|Vtp|=1.8V.
In contrast, at the reference transistor unit 320b, the voltage on the node N1 required for the reference transistor QN11 to operate as a pentode is equal to Vdn+Vtn, where Vtn denotes a threshold voltage of the NMOS transistor QN10. To supply the necessary drain current to the NMOS transistor QN10, the current source PMOS transistor QP3 is also required to operate as a pentode, which causes a voltage drop equal to Vdp+|Vtp|, where Vdp denotes a voltage between the source and the drain. Accordingly, a minimum supply voltage, Vccmin2, at the reference transistor unit 320b comes to Vccmin2=Vdn+Vtn+Vdp+|Vtp|=1.8V+α, which is clearly larger than the minimum supply voltage, Vccmin1, at the reference cell unit 320a. 
As obvious from the above, the conventional reference voltage generator is configured to limit a lower supply voltage because of the reference transistor unit 320b. In a word, even though the bit-line direct-sensing scheme is applied to the sense amp body as described above, the Vccmin can not be improved if the reference voltage generator remains as it is in the art.
The present invention has been made in consideration of such the situation and accordingly has an object to provide a semiconductor memory device using a sense amp circuitry capable of lowering a supply voltage easily.